Aluminum nitride sintered bodies are generally used as a raw material of a substrate for various electronic parts, because they have high thermal conductivity, etc. In this specification, the aluminum nitride sintered body refers to a sintered body comprising aluminum nitride as a major component.
When the aluminum nitride sintered body is used as a substrate of electronic parts, the substrate needs to be prepared into a sheet form having a relatively large area and a small thickness. However, product defects such as deflection of a sintered body substrate have been caused in a typical method for producing an aluminum nitride sintered body. Such deflection causes cracks in the substrate or the like.
Therefore, techniques of preventing the generation of deflection in the substrate have been proposed in the past. For example, in Japanese Patent Application Laid-Open No. 2002-68849, it is stated that an aluminum nitride sintered body having a small thickness and a relatively large area without deflection can be produced by controlling a cooling rate within a predetermined range at sintering. Thus, Patent Application Laid-Open No. 2002-68849 discloses an example of the aluminum nitride sintered body having a thickness of 0.6 mm and maximum dimensions of 100 mm×300 mm.
However, recently there has been a demand for a substrate made of an aluminum nitride sintered body (hereinafter, referred to as an “aluminum nitride substrate”), for example, having a size considerably larger than the above-mentioned dimensions of 100 mm by 300 mm, as a substrate for an electronic part used in a laser printer, copying machine, etc. Furthermore, there is a demand for an aluminum nitride substrate having improved properties in terms of flatness such as controlled warp or waviness in addition to properties such as controlled deflection as mentioned above. With such a conventional method as mentioned above, it has been difficult to produce an aluminum nitride substrate having a large area and a small thickness and yet exhibiting such a controlled warp and waviness, and such substrates have not been available.